The present invention generally relates to biaxially oriented films made from polypropylene and tapes comprising such films.
Commercially available pressure sensitive adhesive tapes are usually provided in a roll form on a tape dispenser (see e.g. U.S. Pat. Nos. 4,451,533 and 4,908,278). Tape dispensers typically have either a metal or plastic serrated cutting blade. xe2x80x9cSeverabilityxe2x80x9d of adhesive tape is defined as the ability to cut or sever a length of tape by pulling the tape over the teeth on the serrated cutting edge of a tape dispenser with a desired amount of energy or work. Severability is also referred to as xe2x80x9cdispensability.xe2x80x9d
It is desired that the severed tape does not chip, sliver, fracture or break in an unpredictable manner (see U.S. Pat. Nos. 4,451,533 and 4,908,278). Such severability is desirable to produce a clearly serrated cut edge on the severed tape strip. Severability is governed primarily by the mechanical properties of the backing of the adhesive tape. Cleanly serrated edges are preferred for aesthetic reasons in applications such as gift wrapping, mending, and the like. The ease with which an adhesive tape can be severed depends on the deformation and resistance to break (toughness) of the tape backing film, also referred to as the substrate. Typically, the substrate is coated or laminated with surface layers to provide an adhesive surface or a matte or writable surface. The greater the energy required to sever the film, the more extensive the resulting damage to the film will be and the less aesthetically appealing the resulting severed edge. In most cases, energy to sever of the tape is governed primarily by the substrate, with little affect by the adhesive or other layers or coatings. This is believed to be due in part to the elastic strain energy built up during the deformation being suddenly and catastrophically released when the film""s inherent strength limit is surpassed. The subsequent failure propagates uncontrollably in a tearing or ripping fashion so that the cut edge does not follow closely the contour of the teeth of the dispenser. The propagation also is directed along the underlying fibrid orientation distribution rather than closely following the contour of the dispenser teeth.
The majority of commercially available biaxially oriented polypropylene film is produced by the flat film or tenter stretching process. Typical tenter processes serve to biaxially stretch films either predominantly simultaneously or predominantly sequentially. Sequential tenter stretching is currently the most widely used biaxial film processing method. Typically, a thick sheet is extruded and rapidly quenched to form spherules having xcex1-morphology with a monoclinic unit cell. This thick sheet is then reheated to a suitable stretching temperature and stretched in a first lengthwise or longitudinal stretching step, followed by a second transverse stretching step and then the film is annealed to produce a flat film having uniform thickness. Simultaneous tenter stretched films comprise a minor part of the film backing market because, although such processes can continuously stretch films in both longitudinal and transverse directions, they have historically proven costly, slow, and inflexible regarding allowable drawing ratios.
Commercially available biaxially oriented polypropylene films are well known for their toughness, moisture stability, good color, silvering resistance, and clarity and have long been used as adhesive tape backings (see U.S. Pat. Nos. 3,241,662 and 3,324,218). Such films are produced typically with so-called xe2x80x9cfilm gradexe2x80x9d isotactic polyproylene resins having high molecular weight and low melt flow rates between about 2-8 grams/10 minutes as measured using ASTM D 1238-95.
High molecular weight polyolefin resins are well known in the art to produce biaxially oriented film having uniform stretch and thickness, without hardbands, wrinkles or other defects. High molecular weight imparts cohesive strength to the cast sheet, allowing uniform stretching at the strain rates typical of commercial biaxial film stretching equipment, especially the high strain rates that can occur in sequentially biaxial orientation film lines.
However, especially in the sequential biaxial orientation case, high molecular weight produces film having high toughness, and adhesives tapes produced from such films tend to elongate significantly before breaking while under load, when renders such tapes extremely difficult to sever, particularly on a plastic-bladed dispenser. The elongation to break for such tapes results in an undesirably large amount of work on the part of the user.
It is known to add other components to polypropylene resin to improve processibility or improve severability for production of adhesive tape backing films.
Small amounts (less than about 10% by weight) of low molecular weight additives like waxes or lubricants can be employed to allow the cast web to be more easily stretched to a film by reducing the internal friction between polymer chains as they slide past each other. WO 97/46369 discloses use of a polypropylene wax additive, by which the execution of a process for preparing a biaxially stretched polypropylene film at high speeds and with high reliability will be possible.
Components like hydrocarbon tackifiers, high Tg polymers and the like can be added to enhance the film brittleness and thereby reduce the work required to sever the film. Such materials may prove difficult to process or impart undesirable stiffness, opacity, off-colors, and higher cost to the finished film.
JP-53-34834 discloses a biaxially oriented polypropylene based adhesive tape with cutting properties formed from a polymer mixture containing 20-80% of polypropylene and 80-20% of a low molecular weight polyolefin. It is further disclosed that in the case when the molecular weight of the low molecular weight polyolefin becomes greater than 20,000 grams/mole, it becomes a material where the cutting properties are significantly deteriorated.
U.S. Pat. No. 3,887,745 discloses a finger-tearible adhesive tape with a film base comprising a layer of polypropylene polymer film which is biaxially oriented and at least one other layer of a polypropylene polymer film which is uniaxially oriented in a transverse direction, the melting point of the uniaxially oriented film being 0.5-10xc2x0 C. higher than the melting point of the biaxially oriented film, and the total thickness of the uniaxailly oriented film being 1.2-5.0 times greater than the total thickness of the biaxially oriented film.
U.S. Pat. No. 4,393,115 discloses a laminate film having hand-cutting properties comprising a biaxially oriented polypropylene layer and a uniaxially oriented polypropylene layer oriented only in the width direction, laminated with a stretched thin surface layer of propylene-ethylene block copolymer. The thickness of the uniaxially oriented layer is in the range between about 1.2 and 5.0 times the thickness of the biaxially oriented polypropylene layer.
U.S. Pat. No. 4,414,261 discloses a severable polypropylene adhesive tape having a base sheet comprising an intermediate layer made of crystalline polypropylene containing from 25 to 35% by weight of a petroleum resin and outer layers made of a crystalline polypropylene laminated on both sides of the intermediate layer, the thickness of the intermediate layer is within a range from 60 to 90% of the base sheet.
U.S. Pat. No. 4,447,485 discloses an adhesive tape with finger-tearability and severability comprising a base sheet of a polypropylene resin containing from 5 to 50% by weight of a methylpentene polymer and a subsidiary layer formed on one or both sides of the main layer composed of crystalline polypropylene having a melting point of at least the melting point of the methylpentene polymer, and a bonding layer of a carboxylic acid-modified polypropylene.
U.S. Pat. No. 5,474,820 disclose a multilayer polypropylene film comprising a base layer of polypropylene and at least one outer layer containing a mixture of HDPE and one or more olefin homopolymers, copolymers or terpolymers and the film has a silk-matte finish. The polypropylene polymer of the base layer has a melting point of at least 140xc2x0 C. and the melt flow index is in the range of 0.5-15 grams/10 minutes.
The present invention provides biaxially oriented substrates comprising isotactic polypropylene compositions. Such biaxially oriented substrates as well suited for use as adhesive tape backings. The tapes and backings of this invention can be severed readily using commercially tape dispensers having metal or plastic cutting teeth to produce cleanly serrated cut edges on the tape. That is, the preferred adhesive tapes and backings described herein have a serrated edge that closely follows the contour of the serrated cutting teeth when tested by the Dispense Testxe2x80x94Metal Blade or the Dispense Testxe2x80x94Plastic Blade described herein.
In one aspect, the present invention provides an adhesive tape comprising a backing and a layer of adhesive on the backing. The backing comprises a biaxially oriented substrate which comprises an isotactic polypropylene composition.
Preferably, the biaxially oriented substrate comprises an isotactic polypropylene composition such that the backing has the following properties taken individually or in any desirable combination:
A) an elongation to break of from 40% to 170%;
B) an energy to server of up to 350 N-cm/cm2 when severed according to The Dispense Testxe2x80x94Metal Blade;
C) an energy to sever of up to 700 N-cm/cm2 when severed according to The Dispense Testxe2x80x94Plastic Blade; and
D) an elongation of up to 4% when severed according to either The Dispense Testxe2x80x94Metal Blade or The Dispense Testxe2x80x94Plastic Blade.
The above characteristics are defined with respect to the backing without adhesive present. It is expected that adhesive has little impact on the characteristics described above.
In one preferred embodiment, the biaxially oriented substrate comprises a polypropylene composition such that the biaxially oriented substrate has the following characteristics taken individually or in any desirable combination:
A) a weight average molecular weight of at least 100,000 grams/mole;
B) a melt flow rate of at least 8 grams/10 minutes, more preferably 12 grams/10 minutes, and most preferably 20 grams/10 minutes;
C) a high isotacticity content greater than about 90% as measured by n-heptane soluble fraction of less than about 15% by weight or having at least 60% isotactic pentads as determined by 13C-NMR analysis; and
D) a density preferably of about 0.86-0.92 grams/cm3, more preferably about 0.88-0.91 grams/cm3.
The biaxially oriented substrate is preferably made from a resin comprising an isotactic polypropylene homopolymer having a melt flow rate greater than 8 grams/10 minutes, more preferably greater than 12 grams/10 minutes, and still more preferably greater than about 20 grams/10 minutes.
The resin also preferably has:
A) a high isotacticity content greater than about 90% as measured by n-heptane soluble fraction content of less than about 15% by weight or having at least 60% isotactic pentads as determined by C-13 nuclear magnetic resonance (NMR) analysis;
B) a weight average molecular weight (Mw) of at least 100,000 grams/mole;
C) a density of 0.86-0.92 grams/cm3, preferably 0.88-0.91 grams/cm3; and
D) a melting point temperature of about 160-166xc2x0 C.
One preferred way to obtain such a resin is to blend:
At least one isotactic polypropylene homopolymer xe2x80x9cAxe2x80x9d having a weight average molecular weight of at least 350,000 grams/mole as determined by size exclusion chromatography and melt a flow rate of about 0.5-8 grams/10 minutes, more preferably about 0.5-xe2x89xa1grams/10 minutes; with
B) at least one isotactic polypropylene homopolymer xe2x80x9cBxe2x80x9d having a weight average molecular weight of at least 100,000 grams/mole as determined by size exclusion chromatography and a melt flow rate greater than 8 grams/10 minutes, and preferably greater than about 20 grams/10 minutes, and more preferably greater than about 50 grams/10 minutes. The practical upper limit for the melt flow rate of Resin B may be about 350-400 grams/10 minutes, although the present invention is not is not limited. The preferred upper limit is about 150 grams/10 minutes. Resin B should have a sufficiently high molecular weight to exhibit molecular chain entanglement behavior, that is to behave as a solid at ambient temperature and exhibit elasticity and melt strength.
Resins A and B each also preferably have:
A) a high isotacticity content greater than about 90% as measured by n-heptane soluble fraction content of less than about 15% by weight or having at least 60% isotactic pentads as determined by C-13 nuclear magnetic resonance (NMR) analysis;
B) a density of 0.86-0.92 grams/cm3, preferably 0.88-0.91 grams/cm3; and
C) a melting point temperature of about 160-166xc2x0 C.
Resins A and B can be melt mixed together and formed into a sheet and biaxially oriented to produce an adhesive tape backing, or arranged as discrete layers in a multilayer film or some combination of two and then biaxially oriented to produce an adhesive tape backing.
The biaxially oriented isotactic polypropylene substrate thus formed from the resin or resins described herein has a melt flow rate greater than about 8 grams/10 minutes and preferably less than about 100 grams/10 minutes, and has improved severability on commercially available adhesive tape dispensers.
The melt flow rates of A and B and the relative amounts of each are selected such that the backing and the biaxially oriented substrate have the desired properties and characteristics described herein.
Additionally, the resin may be a blend of three or more resins that satisfy the requirements of A and B stated above.
When using a blend, resins A and B may be mixed together in any desirable weight combinations, preferably with Resin A present in an amount from about 5 to 80 wt %, more preferably from about 20 to 70 wt %, and with Resin B preferably present in an amount of from 20 to 95 wt %, more preferably from about 30 to 80 wt %.
In one preferred embodiment, the biaxially oriented substrate is a monolayer.
In another preferred embodiment, biaxially oriented substrate is of multilayer construction. In preferred multilayer biaxially oriented substrate comprises discrete layers each made from a resin meeting the requirements of Resin A or B. Additionally, layers not meeting the requirements of Resins A or B may be included either as the outermost layers or within a multilayer construction. In such a case, the composite biaxially oriented substrate preferably has at least 50% of the thickness containing at least 90% isotactic polypropylene. Stated another way, of the layers that together make up the biaxially oriented substrate, the layers comprising at least 90% isotactic polypropylene account for at least 50% of the total thickness of the biaxially oriented substrate. Preferably, the multilayer biaxially oriented substrate has a composite melt flow of greater than about 8 grams/10 minutes, more preferably greater than 12 grams/10 minutes, and most preferably at least 20 grams/10 minutes. The multilayer biaxially oriented substrate has a composite melt flow rate of preferably less than about 100 grams/10 minutes. Furthermore, the backing comprising a multilayer biaxially oriented substrate has the desired severance energy and elongation described herein.
The present invention comprises the desirable adhesive tapes described herein. The present invention also comprises the preferred backings described herein. The present invention also comprises the preferred biaxially oriented substrates described herein, in which case those characteristics described with respect to the backing apply to the biaxially oriented substrate. Such preferred biaxially oriented substrates may be monolayer or multilayer, with monolayer being preferred. The present invention also comprises the preferred resins and blends described herein which may be oriented to provide the preferred biaxially oriented substrates and/or backings described herein.
Certain terms are used in the description and the claims that, while for the most part are well known, may require some explanation. xe2x80x9cArea stretch ratio,xe2x80x9d as used herein, indicates the ratio of the area of a given portion of a stretched film to the area of the same portion prior to stretching. For example, in a biaxially stretched film having an area stretch ratio of 36:1, a given 1 cm2 portion of unstretched film would have an area of 36 cm2 after stretch.
xe2x80x9cBiaxially oriented,xe2x80x9d when used herein to describe a film, indicates that the film has been stretched in two different directions in the plane of the film. Typically, but not always, the two directions are perpendicular. Biaxially oriented films may be sequentially stretched, simultaneously stretched, or stretched by some combination of stimultaneous and sequential stretching. xe2x80x9cSimultaneously biaxially oriented,xe2x80x9d when used herein to describe a film, indicates that significant portions of the stretching in each of the two directions are performed simultaneously.
Molecular weight and molecular weight distribution affect polymer process and mechanical properties. Polymer molecular weight is typically expressed as a weight average molecular weight. Typically, the molecular weight of commercially available polyolefins resins is estimated by measuring the melt flow rate (xe2x80x9cMFRxe2x80x9d), which increases with decreasing molecular weight and hence melt viscosity. Commerical film grade isotactic polypropylene resins for use in biaxial orientation typically have a MFR in the range of about 1-6 grams/10 minutes. At very high MFR values, biaxial orientation of polypropylene resins becomes increasingly difficult because of a lack of melt strength at useful commercial equipment speeds during the stretching operation.
Unless specified otherwise, all values of melt flow rate of inventive resins and films described herein are provided in units of grams/10 minutes measured according to ASTM D 1238-95, Flow Rates of Thermoplastics by Extrusion Plastometer, Procedure B, Condition 230/2.16.
The molecular weight distribution is often characterized by the polydispersity index which typically is the ratio of the weight average molecular weight to the number average molecular weight. The polydispersity index affects processability of polypropylene resins and also the mechanical properties of the resultant biaxially oriented film. The polydispersity index can be determined by measuring the dynamic shear properties of the polypropylene melt (see Proceedings of the 2d World Congress of Chemical Engineering, Montreal, vol. 6, pp. 333-337 (1981)). Preferably, the films of the invention have polydispersity index values between about 2 and 8, more preferably between about 2.5 and 7.
A minimum molecular weight or chain length is required to develop strength in uncrosslinked polymers. The minimum chain length is called the critical chain entanglement molecular weight (Mc) and defines the chain length required for the onset of chain entanglements. Chain entanglements provide topological constraints to chain motion, and allow the polymer to behave like a crosslinked network and bear stress; this ability is sometimes known as green strength or web strength. Below the Mc, uncrosslinked polymer systems have no such constraints and therefore lack elasticity and undergo chain slippage and large scale stress relaxation.
For many polymers, Mc is taken as the inflation point observed for polymers in plots of viscosity against increasing molecular weight (xe2x80x9cViscoelastic Properties of Polymers,xe2x80x9d 3rd ed., J. D. Ferry, p. 242, John Wiley and Sons, NY (1980)). For molecular weights below Mc, polymer viscosity increases with the first power of molecular weight, that is, the viscosity and molecular weight increase monotonically. With decreasing chain length below Mc the polymer behaves first like a wax and finally as a viscous liquid. Above Mc, however, the polymer behaves as a viscoelastic solid and viscosity increases with molecular weight to the 3.4 power; this much greater dependence of viscosity on molecular weight is explained as the effect of entanglements in constraining chain motions and increasing viscosity. Mc can therefore be interpreted as the transition between wax behavior and polymer behavior.
It is often desirable to provide very clear plastic film for use in clear adhesive tapes, packaging, and other applications. The adhesive tape backings herein described in this invention exhibit good clarity and low haze. The biaxially oriented polypropylene film of the present invention displays reduce haze in transmission compared to films which do not meet the requirements of this invention. The oriented films of the present invention preferably are optically clear to the unaided eye under typical interior lighting conditions. More preferably, the films have a haze of less than 5% as measured by ASTM D1003-95.